Power transmission and distribution systems include various insulating components that must maintain structural integrity to perform correctly in often extreme environmental and operational conditions. For example, overhead power transmission lines require insulators to isolate the electricity-conducting cables from the steel towers that support them. Traditional insulators are made of ceramics or glass, but because ceramic insulators are typically heavy and subject to fracturing, a number of new insulating materials have been developed. As an alternative to ceramics, composite materials were developed for use in insulators for transmission systems around the mid-1970's. Such composite insulators are also referred to as “non-ceramic insulators” (NCI) or polymer insulators, and usually employ insulator housings made of materials such as ethylene propylene rubber (EPR), polytetrofluoro ethylene (PTFE), silicone rubber, or other similar materials. The insulator housing is usually wrapped around a core or rod of fiberglass (alternatively, fiber-reinforced plastic or glass-reinforced plastic) that bears the mechanical load. The fiberglass rod is usually manufactured from glass fibers surrounded by a resin. The glass-fibers may be made of E-glass, or similar materials, and the resin maybe epoxy, vinyl-ester, polyester, or similar materials. The rod is usually connected to metal end-fittings or flanges that transmit tension to the cable and the transmission line towers.
Although composite insulators exhibit certain advantages over traditional ceramic and glass insulators, such as lighter weight and lower material and installation costs, composite insulators are vulnerable to certain failures modes due to stresses related to environmental or operating conditions. For example, insulators can suffer mechanical failure of the rod due to overheating or mishandling, or flashover due to contamination. A significant cause of failure of composite insulators is due to moisture penetrating the polymer insulator housing and coming into contact with the fiberglass rod. In general, there are three main failure modes associated with moisture ingress in a composite insulator. These are: stress corrosion cracking brittle-fracture), flashunder, and destruction of the rod by discharge activity.
Stress corrosion cracking, also known as brittle fracture, is one of the most common failure modes associated with composite insulators. The term “brittle fracture” is generally used to describe the visual appearance of a failure produced by electrolytic corrosion combined with a tension mechanical load. The failure mechanisms associated with brittle fracture are generally attributable to either acid or water leaching of the metallic ions in the glass fibers resulting in stress corrosion cracking. Brittle fracture theories require the permeation of water through permeation pathways in the polymer housing and an accumulation of water within the rod. The water can be aided by acids to corrode the glass fiber within the rod. Such acids can either be resident within the glass fiber from hydrolysis of the epoxy hardener or from corona-created nitric acid. FIG. 1 illustrates an example of a failure pattern within the rod of a composite insulator due to brittle fracture. The housing 102 surrounds a fiberglass rod 104. The fracture 108 is caused by stress corrosion due to prolonged contact of the rod with moisture, which causes the cutting of the fibers 106 within the rod.
Flashunder is an electrical failure mode, which typically occurs when moisture comes into contact with the fiberglass rod and tracks up the rod, or the interface between the rod and the insulator housing. When the moisture, and any by-products of discharge activity due to the moisture, extend a critical distance along the insulator, the insulator can no longer withstand the applied voltage and a flashunder condition occurs. This is often seen as splitting or puncturing of the insulator rod. When this happens, the insulator can no longer electrically isolate the electrical conductors from the transmission line structure.
Destruction of the rod by discharge activity is a mechanical failure mode. In this failure mode, moisture and other contaminants penetrate the weather-shed system and come into contact with the rod resulting in internal discharge activity. These internal discharges can destroy the fibers and resin matrix of the rod until the unit is unable to hold the applied load, at which point the rod usually separates. This destruction occurs due to the thermal, chemical, and kinetic forces associated with the discharge activity.
Because the three main failure modes can mean a loss of mechanical or electrical integrity, such failures can be quite serious when they occur in transmission line insulators. The strength and integrity of composite insulators depends largely on the intrinsic electrical and mechanical strength of the rod, the design and material of the end fittings and seals, the design and material of the rubber weather shed system, the attachment method of the rod, and other factors, including environmental and field deployment conditions. As stated above, many composite insulator failures have been linked to water ingress into the fiberglass material comprising the insulator rod. Since all three failure modes—brittle fractures, flashunder, and destruction of the rod by discharge activity, occur in the insulator rod, they are hidden by the housing and cannot easily be seen or perceived through casual inspection. For example, simple visual inspection of an insulator to detect failure due to moisture ingress requires close-up viewing that can be very time consuming, costly, and generally does not yield a definitive go or no-go rating. Additionally, in some cases, detection of rod failure through visual inspection techniques may simply be impossible. Other inspection techniques, such as daytime corona and infrared techniques can be used to identify conditions associated with discharge activity, which may be caused by one of the failure modes. Such tests can be performed some distance from the insulator, but are limited in that only a small number of failure modes can be detected, the discharge activity must be present at the time of inspection to be detected. Furthermore, for this type of inspection, a relatively high level of operator expertise and analysis is required.
It is desirable, therefore, to provide improved inspection techniques for composite insulators or any other type of composite system with external protective coverings that detect failure modes associated with exposure of the interior structure to moisture by yielding a migration path from the inside of the insulator to the exterior surface.
It is further desirable to provide composite insulators that provide early warning of impending failure due to stress corrosion, flashunder, or destruction of the rod by discharge activity, and that allow inspection from a distance and without the need for the actual manifestation of failure symptoms.
It is desirable to provide an automated inspection of composite insulators in the field by instrument-based scanning and image processing.